Packages for housing semiconductors are well known in the art. However, there are ongoing efforts to improve a variety of parameters associated with semiconductor packages. Important semiconductor package parameters that require optimization include thermal dissipation, die stress, package hermeticity, and package cost. A number of these factors are addressed in a set of patents (U.S. Pat. Nos. 4,888,449; 4,897,508; 5,023,398; 5,073,521; 5,013,871; 4,939,316; 4,461,924; 5,015,803; 5,043,534; 5,066,368) issued to the Olin Corporation of New Haven, Conn. Each of the foregoing patents is expressly incorporated by reference herein. The recited patents describe techniques for packaging semiconductors in a metal housing.
FIG. 1 illustrates one technique for packaging a semiconductor in a metal housing. Specifically, FIG. 1 illustrates a metal semiconductor package 20 with an internal adhesive seal. The package 20 includes a metal base 22 formed of a metal material such as anodized aluminum. The package 20 also includes a metal cap 24, which may also be formed of anodized aluminum. A standard lead frame 26 is positioned between the base 22 and the cap 24.
Prior to positioning the cap 24 over the lead frame 26, standard techniques are used to couple a semiconductor die 28 to the base 22 with a die adhesive 30 (in an alternate configuration, the semiconductor die 28 can be connected to the lead frame 26). Afterwards, in accordance with any of a number of well known techniques, bond wires 32 are connected between lead fingers of the lead frame 26 and bond pads (not shown) of the semiconductor die 28.
An internal adhesive seal 34 is used to couple the base 22 and the cap 24. In particular, a base-lead frame seal 34A is used to couple the base 22 to the lead frame 26, and a cap-lead frame seal 34B is used to couple the cap 24 to the lead frame 26.
There are a number of problems associated with the metal semiconductor packages of the type illustrated in FIG. 1. For example, the semiconductor die 28 will only be protected if a perfect adhesive seal is formed. A perfect seal is difficult to obtain in most commercial processes. For instance, difficulties may arise when the base 22 and the cap 24 are of slightly different sizes or planarity, thereby resulting in a sealing mismatch. In addition, it is difficult to insure that the adhesive is uniformly applied along the perimeter of the base 22 or cap 24. Refinements in the manufacturing process to improve alignment, to diminish discrepancies in base and cap sizes, or to improve the uniformity of adhesive application result in increased expense. Also adding to the expense associated with prior art metal semiconductor packages utilizing internal adhesive seals is the requirement that they be tested to insure that an environment-retarding seal has actually been obtained. As used herein, the term environment-retarding seal refers to a substantial barrier to moisture and gases. An environment-retarding seal is not as complete as the hermetic seal provided by metal, glass, or a ceramic material.